1. Field of the Invention
The present invention relates to a display and an array substrate and, more particularly, to an active matrix organic EL (electroluminescence) display and an active matrix substrate used therefore.
2. Description of the Related Art
Demand for flat panel displays represented by a liquid crystal display is growing sharply because of their advantages such as low profile, low weight, and low power consumption as compared to cathode-ray-tube displays. Especially, active matrix displays which use pixels having a memory effect to hold a video signal are used in various devices, such as portable information equipments, because a satisfactory display quality can be obtained.
In recent years, of flat panel displays, organic EL displays as self-emission displays which can increase the response speed and viewing angle as compared to liquid crystal displays have intensively been developed.
FIG. 1 is a circuit diagram showing an example of the pixel circuit of a conventional organic EL display. This pixel circuit is disclosed in U.S. Pat. No. 6,373,454B1 and operates as described below.
First, a switch Sw2 is opened. In this state, switches Sw1 and Sw3 are closed to supply a desired video signal current Iin to a transistor Dr. At this time, since the transistor Dr is diode-connected by the switch Sw1, the gate-to-source voltage of the transistor Dr is set such that the magnitude of the current that flows between the source and drain equals the current Iin. After that, the switches Sw1 and Sw3 are opened. The gate-to-source voltage of the transistor Dr, which is set in correspondence with the current Iin, is held by a capacitor C1. In the above way, the write period is ended.
Next, the switch Sw2 is closed to connect an organic EL element OLED to the drain of the transistor Dr. Since the gate-to-source voltage of the transistor Dr is set as descried above, a current having almost the same magnitude as the current Iin flows to the organic EL element OLED. Accordingly, the light emission period starts. Note that the light emission period continues until the next write period starts.
In the above-described display method, the gate-to-source voltage is ideally held at a constant level during the light emission period. However, if the switch Sw1 is incompletely turned off, charges can move between the gate and drain of the transistor Dr, and the gate-to-source voltage varies. As a result, image display corresponding to the written video signal may be difficult. For example, the luminance of dark display pixels may increase. In extreme cases, the dark display pixels may visually be recognized as bright defects.
This problem not only exists in an organic EL display whose pixels use the circuit shown in FIG. 1. More specifically, the above problem can also be exist in an organic EL display whose pixels use not the circuit which writes a video signal by a current signal but a circuit which writes a video signal by a voltage signal.